I. Field of the Invention
The present invention relates to methods of optical data storage and more specifically to optical storage by photochemical hole burning.
II. Description of Related Art
Optical data storage systems have become highly popular for computer mass storage. These systems offer high storage density with high data rates, rapid random access to data, potential archival properties and relatively low media cost.
The best known optical data storage systems are those which follow the video disk technology of transferring information onto a mechanical configuration of pits stamped into a metal disk. The first type of system involves read only memory (ROM).
A second type of optical memory uses a programmable disk scheme which is "write once/read many times". This system permits the user to preserve files by burning pits into the disk with a laser which can later be used, at a lower power, to detect the pit. Some of these systems provide erasability, but erasable disks have the disadvantages of low signal to noise ratio and degradation of the recording quality after consecutive reads on the same location, causing an increase in the bit error rate.
Typically, optical data storage systems use lasers of the same wavelength for reading and writing, (e.g., semiconductor diode lasers at 830 nm, He-Ne lasers at 633 nm, argon lasers are 488 nm and He-Cd lasers at 422 nm). The power of the read beam is reduced so as not to degrade the recording medium. Because every pit in the disk can be detected by the laser used, adequate spacing must be permitted between the pits to avoid overlap of signals picked up by the reader. Thus, the density of data is limited based on considerations of beam width and tracking accuracy.
A method of significantly improving the data packing density is to use different wavelengths of light to activate a photochemically active medium which is sensitive to discrete wavelengths. This photochemically-active medium is usually set in a glass or polymer matrix. The phenomenon of "photochemical hole burning" involves excitation of a fraction of the molecules of photoactive material, usually in the form of a crystal, so that the excited modules no longer contribute to absorption of the laser wavelength, resulting in a hole or dip in the absorption line when the location is illuminated by light of the same wavelength. This excitation is induced by exposure of the photo-active material to narrow-band optical radiation tuned to a frequency within the inhomogeneously-broadened zero-photon line of the material the mechanism responsible for the hole burning is thought to be ionization caused by electron tunneling from photoexcited centers to nearby traps.
Approximately 1,000 discrete, resolvable holes can be burned at each spacial location, with a theoretical possibility of 10.sup.11 bits/cm.sup.2.
Clearly, one limitation on achieving the theoretical goal for photochemical hole burning is the ability to control a laser sufficiently t repeatably select discrete wavelengths of light.
Researchers in the field have proposed the use of tunable dye lasers which are capable of tight control, but are complex and bulky, contributing to packaging problems, and are expensive. Other researchers have used diode lasers which have been tuned by periodically ramping the injection current of the laser to scan repetitively over the spectral region. This scanning method is somewhat haphazard, lacking control and repeatability and, as is well known, diode lasers are subject to shifts in output wavelength as a function of temperature. Typically, this technique allows addressing of only about 100 separate frequency channels.
It would be desirable to have a laser for optical data storage systems using photochemical hole burning which avoids the size, packaging and expense problems of tunable dye lasers, yet is more controllable and less temperature sensitive than tunable diode lasers, thereby permitting a greater number of frequency channels within the active range of the photoactive medium. It is to this objective that the present invention is directed.